The present invention concerns improvements in the construction and operation of load handling mechanisms or manipulators which are used for hoisting, positioning, and otherwise handling of loads, such as workpieces, within a given working space having predetermined dimensions and also serve for loading or unloading parallel wall spaces such as shelves. More particularly, the invention relates to a load handling device or manipulator which functions according to pantograph principles having a parallelogram linkage which enables movement of the load in a horizontal direction in a very simple manner.
In devices of this pantograph type, a horizontal guideway directs movement of the load in an axial horizontal direction. A pivot bearing enables horizontal rotary motion of the load handling device around a vertical axis at a given vertical evaluation from a floor, ceiling, or carrier. Since the geodetic elevation of the load is not altered given movement of the load in the horizontal direction, this movement theoretically requires almost no work. The vertical movement and positioning of a load has been typically accomplished by means of a pressure agent which acts on a piston in a load cylinder. The piston has a piston rod projecting out of the cylinder, such that upward or downward movement of the load is due to the admission or discharge of the pressure agent flow in the cylinder and the load is held at a given elevation given a cylinder closed to such flow.
A pressure balance control device has been employed in the operation of such pantograph load handling devices. In such a control device the pressure in the load cylinder is compared to a constant reference pressure that is predetermined for each load and, depending on the pressure differential, pressure agent is supplied to or discharged from the load cylinder by means of a balancer valve such that pressure agent is supplied into the load cylinder when the cylinder pressure is lower than the reference pressure and pressure agent is discharged when the cylinder pressure is higher than the reference pressure. The existence of a differential between load cylinder pressure and reference pressure usually arises from manual lifting or lowering of the load. The operator thus needs to exert such force on the load that the friction in the load cylinder is overcome and this pressure differential becomes great enough for the balancer valve to operate in order for him to vertically move the load. Load cylinder friction is the result of friction between the piston seal and cylinder wall and between the piston rod and its guide means.
The force exerted to overcome load cylinder friction must be so high that a sensitive and accurate movement that does not present fatigue problems is typically not possible.
Another disadvantage to piston-type load cylinders heretofore employed in pantograph load handling devices is that the piston rod must be precisely guided in a straight line motion in order to achieve a faultless interaction of the piston seal with the cylinder wall. Yet, the load cylinder and piston rod are connected to different parts of the load handling device which are relatively movable and involve still further guide devices. Thus, the guide connection between the cylinder and piston rod must therefore be flexible, and this has necessitated that the guide connection be large and of considerable overall length making it relatively expensive.